Polypropylene is a well-known article of commerce, and is utilized in a wide variety of applications which are well known to those of ordinary skill in the art. Polypropylene is utilized widely in many fiber, fabric, or similar product applications. However, it is generally deficient in applications that require high softness such as nonwoven fabrics for disposable garments and diapers. For such soft-end use fiber and fabric applications, macromolecules with a statistical placement of propylene and ethylene monomer units (hereinafter random copolymers) have come into use since they can be processed into fibers and fabrics that exhibit improved softness and drape characteristics in comparison to fibers and fabrics made from homopolymer polypropylene.
Random copolymers are made by adding small amounts of ethylene in the reacting medium comprising propylene and a catalyst that is capable of randomly incorporating the ethylene monomer into the macromolecule chain, to thereby reduce the overall crystallinity and rigidity of the macromolecule. Random copolymers, because of their lower crystallinity and rigidity, are preferred over homopolymer polypropylene in fiber and fabric applications that require enhanced softness. However, a number of practical limitations have limited the application of random copolymers in soft-end fiber and fabric uses. One limitation has been the inability of polypropylene manufacturers to economically incorporate ethylene at levels generally above about 5% by weight of the random copolymer. Another limitation has been the inability of existing fiber and fabric processes to economically draw fine diameter fibers and good coverage fabrics from conventional high ethylene content random copolymers and in particular random copolymers having an ethylene content greater than about 3% by weight. Coverage is defined as weight of polymer per unit area of the fabric. It is often the most important fabric parameter, since it is related to the yield and, thus the area cost. These and other limitations will become apparent from the following discussion of a typical spunbond process.
Random copolymers have long been used in the making of nonwoven spunbonded fabrics. In a typical spunbond process a random copolymer resin in granular or pellet form is first fed into an extruder, wherein the resin simultaneously is melted and forced through the system by a heating melting screw. At the end of the screw, a spinning pump meters the melted polymer through a filter to a die (hereinafter the spinneret) having a multitude of holes (hereinafter capillaries) where the melted polymer is extruded under pressure through the capillaries into fibers. The fibers exiting the spinneret are being solidified and drawn into finer diameter fibers by high-speed air jets. The solidified fibers are laid randomly on a moving belt to form a random fibrous, mesh-like structure known in the art as a fiber web. For optimum softness and drape characteristics, solidification of the fibers must occur before the fibers come into contact with one another, in order to prevent the fibers form sticking together. This phenomenon, of the fibers sticking together, ultimately results in a more rigid and less soft fabric. After web formation, the web is then bonded to achieve its final strength by pressing it between two heated steel rolls (hereinafter the thermobond calender).
The ethylene content of the random copolymer that is used to make the fibers is one of the parameters that effect the softness feel and drape characteristics of the spunbonded fabric. It has long been recognized that softer spunbonded fabrics could be produced by raising the amount of ethylene content in the random copolymer. Generally the greater the ethylene content of the copolymer is, the less rigid and the more elastic each fiber becomes, thus imparting a softer feel characteristic to the fabric itself. However, fibers made from random copolymers having increasingly higher ethylene content take longer to solidify with the result that they tend to stick together forming coarser fibers before solidification occurs. The result of this phenomenon is, inter alia, that the fabric's uniformity, coverage (basis weight per unit area) and drape/handle characteristics suffer. The fabric becomes more rigid and less soft. Although, this problem could perhaps be somewhat alleviated by lowering the throughput rate, to allow more time for these resins to solidify before they come into contact, it generally becomes uneconomical to process random copolymers having an ethylene content greater than about 3.5% by weight of the total polymer, because of the generally very low throughput rate required to prevent the fibers from sticking together.
Moreover, random copolymers having an ethylene content greater than about 5% by weight have not generally been feasible to be produced in liquid reactor or hybrid reactor technologies. The term "liquid reactor technology" as used herein encompasses slurry polymerization processes wherein polymerization is conducted in inert hydrocarbon solvents and bulk polymerization processes wherein polymerization is conducted in liquefied propylene. The term "hybrid reactor technology" as used herein refers to polymerization processes comprising one or more liquid reactor systems followed by one or more gas phase reactors. Liquid only and hybrid reactor systems account for the most part of polypropylene manufacturing capacity worldwide. In a liquid reactor system, the liquid hydrocarbon solubilizes the atactic portion of the polymer, the level of which is enhanced by the high incidence of ethylene monomer in the polymer chain. The atactic material is tacky and creates flowability problems in the downstream equipment as soon as the liquid hydrocarbon is vaporized. Because of this phenomenon, ethylene incorporation in the random copolymer is limited to a maximum of about 5% by weight, in a liquid reactor system. Above that level, tacky copolymer granules would agglomerate and/or stick to the metal walls of the process equipment generally resulting in the clogging thereof.
Processes employing hybrid reactor technology have been widely used in the production of thermoplastic olefin resins (hereinafter TPO), but generally not in the production of random copolymers. A typical TPO resin, as per U.S. Pat. Nos. 3,806,558, 4,143,099 and 5,023,300, comprises a first homopolymer or random copolymer component and a second rubber-like component known as an olefin copolymer elastomer. Generally, it has been a widely held belief, among persons skilled in the TPO art, that lowering the ethylene content of the elastomer component below about 30 to 40% by weight range would result in severe fouling and shutdown of the gas phase reactor. Thus, conventional, TPO resins albeit of a high ethylene content, are generally not suitable for typical random copolymer applications such as fiber making, since the elastomer component of a TPO resin contains large amount of ethylene that renders it immiscible with the homopolymer or random copolymer portion and the higher ethylene elastomeric portion would be unsuitable for fiber extrusion.
Therefore, it has been highly desirable to develop a polypropylene based resin having an ethylene content higher enough to allow the making of softer fibers and fabrics without the processing and physical drawbacks of conventional high ethylene random copolymers and/or TPO resins.